Creutzfeldt-Jakob Disease (CJD) is a human dementia caused by a slow virus. Because clinical symptoms are not expressed for many years after infection, the risk of iatrogenic infection from asymptomatic donors is unknown. Buffy coat studies have suggested a more widespread prevalence of CJD infection than previously appreciated, but the current rodent assay for infection is too lengthy to be informative for a large population study. 1) We seek to exploit our established rodent models of CJD to understand more about the pathogenesis of the disease and the mechanisms of viral clearance. In this context, specific cell types in the brain appear to be recruited to limit viral replication at this site. Such features have become more apparent in a new rodent model of CJD with prolonged incubation times and unusual pathological features. Interestingly, some of the responses to thwart viral replication may also accelerate neuronal damage using pathways commonly involved in other late-onset dementias. 2) We are developing tissue culture and other models to elucidate cell-type specific susceptibility factors. These models may yield more sensitive and rapid assays for the virus. They should also help to clarify and distinguish between genetic and infectious dementias that show similar phenotypic changes in the brain. 3) Rapid and reliable assays of the human virus, present at low titers in peripheral tissues, are needed. A number of experiments indicate the virus contains an RNA of substantial size (>1kb) protected by a nucleocapsid coat. Modified subtractive approaches are being used to recover segments of the viral genome from highly purified CJD preparations. Candidate fragments of 200-500 bases that are not present in the host genome have been visualized. These are currently being analyzed. If the method is as robust as it appears to be, it can be applied for the discovery of other unsuspected or new RNA viruses that participate in late-onset dementias.